It is expected that a system will support one or more physical
networks and for each physical network supported the appropriate
protocols from the above list must be supported. That is, it is
elective to support any particular type of physical network, and for
the physical networks actually supported it is required that they be
supported exactly according to the protocols in the above list. See
also the Host and Gateway Requirements RFCs for more specific
information on network-specific ("link layer") protocols.